The present invention concerns a measuring device for contactless detection of a ferromagnetic object.
Measuring devices of this nature generally comprise an arrangement of a magnet structure and an integrated control circuit chip having a Hall element, whereby the chip is located on one end of the magnet structure and in the magnetic field that is formed by this magnet structure. The Hall element produces an electrical signal based on the strength of the magnetic field perpendicular to the plane of the Hall element. If a ferromagnetic object now approaches the Hall element, the strength of the magnetic field perpendicular to the Hall element changes. As a result, the distance of the ferromagnetic object away from the Hall element can be represented by an electrical signal produced by the Hall element. Hall sensors of this type are used to detect rotational speeds or certain positions of toothed trigger wheels (gears) in motor vehicles, e.g., for an antilock braking system or engine management.
For example, to realize a small offset field, measuring devices having two closely adjacent Hall elements were proposed that are interconnected electrically in such a fashion that they cancel each other out in the normal state. This makes it possible to produce a good signal, but the two Hall elements must have absolutely identical behavior. This is difficult to achieve in series production.
Furthermore, a Hall-effect sensor is made known in DE-196 22 561, in the case of which a Hall element located on a control circuit chip is situated between a trigger wheel and a magnet structure. The magnet structure is designed so that a north magnetic pole and a south magnetic pole are located next to each other and both of them are located adjacent to the Hall element.
Furthermore, a magnetic field source is made known in EP-0 273 129, in the case of which an annular magnet is provided as the permanent magnet. A Hall generator is associated with the opening of the annular magnet in such a fashion that the axis of the Hall generator and the axis of the annular magnet basically coincide, and, when the magnetic circuit is open, the Hall generator is located in a space having minimal magnetic induction formed by field displacement within the annular magnet.